Potentiometric sensors are used for detecting chemical or biochemical compounds in a solution. Such potentiometric sensors are electrochemical sensors that generate voltages that scale with the concentration of an ion to be determined. A pH sensor is an example of such a potentiometric sensor. A potentiometric sensor usually comprises an ion-selective electrode and a reference electrode. The reference electrode has a potential which can be used as a reference potential for the ion-selective electrode. The potential difference between the ion-selective electrode and the reference electrode is a measure for the concentration of the compound for which the ion-selective electrode is sensitive. An important requirement is that the potential of the reference electrode is stable and constant.
In operation, the reference electrode is immersed, together with the ion-selective electrode, in the solution under test. For proper operation, the interfacial potential of the reference electrode should be independent of the solution composition.
A commonly used type of reference electrode is a silver chloride electrode (Ag/AgCl). This electrode has a fixed potential when in contact with a reservoir with a fixed chloride concentration, such as 3 Molar KCl (3M KCl). An example thereof is shown in FIG. 1. FIG. 1 shows a prior art reference electrode comprising an Ag/AgCl wire 110 immersed in a reservoir 120 containing a predetermined chloride concentration, for instance 3 Molar KCl (the reference electrolyte solution). An electrolyte bridge or a porous ceramic plug separates the reference electrolyte solution from the solution which needs to be measured. In the example reference electrode of FIG. 1, the electrolyte bridge is a porous frit (salt bridge) 130 separating the inner reservoir and the bulk solution. Ions can still pass through this junction; therefore an ionic contact forms. The potential of the Ag/AgCl electrode depends on the chloride concentration in the reference electrolyte solution. The electrolyte bridge or the porous ceramic plug prevents chloride ions from instantaneously migrating between the liquid of the reference electrode and the solution. If the reference electrode is immersed in a solution with a different chloride concentration, chloride ions will leach out which leads to a change in chloride concentration in the reservoir and therefore drift of the reference electrode. Depending on the volume of the reference electrolyte solution, these migrations will cause the chloride concentration in the reference electrolyte solution to change, thereby changing the reference voltage. As the reservoir is rather large in the example of FIG. 1, drift is limited. One way of stabilizing the reference voltage is to increase the volume of the reference electrolyte solution. This is, however, not possible in microfabricated reference electrodes since the size is limited in these electrodes.
Microfabricated reference electrodes typically comprise a planar electrode, made by photolithography or screen-printing techniques covered by a hydrogel, such as agarose or polyhydroxyethylmethacrylate (pHEMA). An example disclosed by Simonis et. al. “Miniaturised reference electrodes for field-effect sensors compatible to silicon chip technology,” Electrochimica Acta 51, vol. 51, issue 5, 10 Nov. 2005, pp. 930-937, is shown in FIG. 2. FIG. 2 shows a printed circuit board 216 comprising a stack of a silicon layer 222, covered with a SiO2 layer 220, covered with an Ag/AgCl layer 218. The Ag/AgCl layer is covered by a hydrogel 214 which comprises KCl and may be, for instance, agarose (agar)+KCl or polyhydroxymethylmethacrylate (pHEMA)+KCl. The hydrogel layer 214 is sealed with a PVC layer 210 and the PVC layer is covered with a nafion or cellulose nitrate layer 212.
The covered hydrogel layer 214, 210 is sealed at its sidewalls by means of a sealing (e.g., an O-ring) 208. The sealing 208 is sealed against the Ag/AgCl layer. Outside the sealing 208 an epoxy resin 204 is applied, thus screening the outside part of the Ag/AgCl electrode (the part not covered by the sealing 208 and the hydrogel 214) from the environment. Silicone 206 is applied partly covering the epoxy resin 204, the sealing 208, and the nafion or cellulose nitrate 212. An electrical contact 202 is in contact with the Ag/AgCl electrode 218 and allows for measuring the potential of the electrode 218.
Increasing the stability of miniaturized reference electrodes is preferably done without increasing the volume of the reference electrolyte solution. Many studies on miniaturized reference electrodes have focused on the composition of the reference electrolyte solution (e.g., a hydrogel), and covering it with membranes to slow down the out-diffusion of chloride ions. Besides these efforts there is still room for improving the stability of miniaturized reference electrodes.